1. Field of the Technology
The present disclosure is directed to methods for straightening high strength titanium alloys aged in the α+β phase field.
2. Description of the Background of the Technology
Titanium alloys typically exhibit a high strength-to-weight ratio, are corrosion resistant, and are resistant to creep at moderately high temperatures. For these reasons, titanium alloys are used in aerospace and aeronautic applications including, for example, landing gear members, engine frames and other critical structural parts. Titanium alloys also are used in jet engine parts such as rotors, compressor blades, hydraulic system parts, and nacelles.
In recent years, β-titanium alloys have gained increased interest and application in the aerospace industry. β-titanium alloys are capable of being processed to very high strengths while maintaining reasonable toughness and ductility properties. In addition, the low flow stress of β-titanium alloys at elevated temperatures can result in improved processing.
However, β-titanium alloys can be difficult to process in the α+β phase field because, for example, the alloys' β-transus temperatures are typically in the range of 1400° F. to 1600° F. (760° C. to 871.1° C.). In addition, fast cooling, such as water or air quenching, is required after α+β solution treating and aging in order to achieve the desired mechanical properties of the product. A straight α+β solution treated and aged β-titanium alloy bar, for example, may warp and/or twist during quenching. (“Solution treated and aged” is referred to at times herein as “STA”.) In addition, the low aging temperatures that must be used for the β-titanium alloys, e.g., 890° F. to 950° F. (477° C. to 510° C.), severely limit the temperatures that can be used for subsequent straightening. Final straightening must occur below the aging temperature to prevent significant changes in mechanical properties during straightening operations.
For α+β titanium alloys, such as, for example, Ti-6Al-4V alloy, in long product or bar form, expensive vertical solution heat treating and aging processes are conventionally employed to minimize distortion. A typical example of the prior art STA processing includes suspending a long part, such as a bar, in a vertical furnace, solution treating the bar at a temperature in the α+β phase field, and aging the bar at a lower temperature in the α+β phase field. After fast quenching, e.g., water quenching, it may be possible to straighten the bar at temperatures lower than the aging temperature. Suspended in a vertical orientation, the stresses in the rod are more radial in nature and result in less distortion. An STA processed Ti-6Al-4V alloy (UNS R56400) bar can then be straightened by heating to a temperature below the aging temperature in a gas furnace, for example, and then straightened using a 2-plane, 7-plane, or other, straightener known to a person of ordinary skill. However, vertical heat treatment and water quenching operations are expensive and the capabilities are not found in all titanium alloy manufacturers
Because of the high room temperature strength of solution treated and aged β-titanium alloys, conventional straightening methods, such as vertical heat treating, are not effective for straightening long product, such as bar. After aging between 800° F. to 900° F. (427° C. to 482° C.), for example, STA metastable β-titanium Ti-15Mo alloy (UNS R58150) can have an ultimate tensile strength of 200 ksi (1379 MPa) at room temperature. Therefore, STA Ti-15Mo alloy does not lend itself to traditional straightening methods because the available straightening temperatures that would not affect mechanical properties are low enough that a bar composed of the alloy could shatter as straightening forces are applied.
Accordingly, a straightening process for solution treated and aged metals and metal alloys that does not significantly affect the strength of the aged metal or metal alloy is desirable.